A nickel-cadmium storage battery has been commonly used as an alkaline storage battery. However, a nickel-hydrogen storage battery that uses a hydrogen absorbing alloy for a negative electrode has recently received attention because it has a high capacity as compared to a nickel-cadmium storage battery, and it is excellent from the view point of protecting the environment because it does not use cadmium.
Nickel-hydrogen storage batteries have been used for portable equipment. It is required to improve the batteries so that they are highly efficient.
As an alloy to be used for the negative electrode of a nickel-hydrogen storage battery, a rare earth-nickel hydrogen absorbing alloy having a crystal structure of the CaCu5 type as the main phase, a Laves phase hydrogen absorbing alloy containing Ti, Zr, V and Ni, and the like, have commonly been used.
However, such hydrogen absorbing alloys do not have sufficient hydrogen absorbing capacity, and it is difficult to increase the capacity of the nickel-hydrogen storage battery.
A rare earth-nickel hydrogen absorbing alloy containing Mg, such as an alloy represented by Ln1-xMgxNi2 or Mg2LnNi9, or a hydrogen absorbing alloy containing at least one element selected from Li, V, Nb, Ta, Cr, Mo, Mn, Fe, Co, Al, Ga, Zn, Sn, In, Cu, Si, P and B in addition to Mg has recently been proposed (for example, Japanese Patent Laid-open publication No. 2001-316744).
However, the hydrogen absorbing alloy containing Mg has a generally larger hysteresis as compared to the rare earth-nickel hydrogen absorbing alloy having a crystal structure of the CaCu5 type as the main phase, and when hydrogen gas is reacted with the hydrogen absorbing alloy at a variable pressure at a constant temperature to absorb and release hydrogen (vapor-solid cycles), an amount of hydrogen remaining in the hydrogen absorbing alloy gradually increases. If such hydrogen absorbing alloy is used for an electrode of an alkaline storage battery and the battery is repeatedly charged and discharged, the capacity of the battery is gradually reduced and the cycle life of the battery is deteriorated.
The hydrogen absorbing alloy described above is easily oxidized. When the alloy is used for a negative electrode of an alkaline storage battery, the alloy is easily oxidized by an alkaline electrolyte. Such oxidation consumes the alkaline electrolyte causing a shortage of the electrolyte which increases resistance in the battery and causes a reduction of the cycle life of the battery.